The wealth of hominin fossils from Sima de los Huesos is helping reveal trends in Neandertal evolution. | AAAS/Carla Schaffer

Neandertals' trademark facial features appeared first in their evolution, while other defining features in their skulls came along later, researchers studying an impressive collection of skulls in a Spanish cave report in the 20 June issue[4] of the journal Science.

These new data from the Sima de los Huesos site will help paleontologists better understand hominin evolution during the Middle Pleistocene, a period in which the path of hominin evolution has been controversial.

About 400,000 to 500,000 years ago, a group of archaic humans split off from other groups living in Africa and East Asia, ultimately settling in Eurasia where they evolved characteristics that would come to define the Neandertal lineage. Several hundred thousand years after that, modern humans — who had evolved in Africa — settled in Eurasia, too. They interbred with Neandertals, but even then showed signs of reproductive incompatibility. Because of this, modern humans eventually replaced Neandertals.

The degree of divergence between Neandertals and modern humans over such a short period of time has surprised scientists. Why did Neandertals differentiate so quickly from other early hominins? What pattern of changes did Neandertals undergo? The team that uncovered the Sima skulls hopes their find will help answer some of these questions.

"The nature of the evolutionary process that gave rise to Neandertals has been discussed for decades, with an important question being whether the 'neandertalization process' involved all regions of the skull from the beginning," said co-author Ignacio Martínez, a professor of paleontology at the University of Alcalá. "Or if on the contrary, there were various stages in this process that affected different parts of the skull at different times."

"With the skulls we found here," Martínez continued, "it is possible to characterize the cranial morphology of a human population of the European Middle Pleistocene for the first time."

The Sima de los Huesos site in Atapuerca, Spain has been excavated continuously since 1984, yielding nearly 7000 hominin fossils. "What makes the Sima de los Huesos site unique," said lead author Juan-Luis Arsuaga, a professor of paleontology at the Complutense University of Madrid, "is the extraordinary and unprecedented accumulation of hominin fossils there; nothing quite so big has ever been discovered for any extinct hominin species — including Neandertals."

Arsuaga and his team were delighted to work on this effort. "Finding a single tooth is a great success in any other site of comparable age, so imagine what it is like to painstakingly reconstruct 17 skulls," he said. "It's like finding a treasure."

The 17 skulls uncovered at Sima de los Huesos belong to a single population of a fossil hominin species. Some have been studied before, but seven are presented for the first time in this study, and six are more complete than ever before. With this range of samples, the researchers made progress characterizing the population's defining features.

In the Sima skulls, Martinez and colleagues identified Neandertal-derived characteristics in the face and teeth, but not elsewhere; the braincase, for example, showed features of more primitive hominins.

"Finding a single tooth is a great success in any other site of comparable age, so imagine what it is like to painstakingly reconstruct 17 skulls. It's like finding a treasure."

Juan-Luis Arsuaga

"Based on this morphology," Arsuaga said, "we think the Sima people were part of the Neandertal clade, although not necessarily direct ancestors to the classic Neandertals." They were part of an early European lineage comprising Neandertals, he suggested, but more primitive than the later Pleistocene variety. "Other fossils of the same geological period are different," Arsuaga added. "They don´t fit the Sima pattern. This means there was a lot of diversity among different populations in the Middle Pleistocene."

Critically, many of the Neandertal-derived features the researchers observed in the Sima skulls were related to chewing. "It seems these modifications had to do with an intensive use of the frontal teeth," Arsuaga explained.

The work of his team suggests that facial modification was the first step in Neandertal evolution, supporting a model in which Neandertal features evolved in stages. In past, the validity of this model has been hard to evaluate. Scientists have needed an accurate picture of European populations around 400,000 years ago, during the early stages of the Neandertal lineage, but this has been a challenge because the European fossil record is isolated and dispersed, with remains from disparate times.

Study co-author Lee Arnold, Australian Research Council Future Fellow at the University of Adelaide, applied a special suite of dating techniques to determine the age of the Sima fossils. "Having resolved the chronology of Sima de los Huesos," he said, "the next step is to improve the dating of other similarly aged paleoanthropological sites in the region. This is a critical step for unravelling the complex pattern of human evolution across Eurasia."